It has long been known that unique combinations of properties can be obtained in many articles by producing such articles as composite materials, so as to utilize desirable properties contributed by dissimilar materials incorporated in the composite structure. Examples which quickly come to mind include glass-fiber reinforced plastic articles widely used in marine and aviation applications such as boats, carbon fiber reinforced articles such as golf club shafts and many other articles, composite copper-cored coins such as the United States dime, quarter, etc. It has been recognized that special combinations of properties can be provided by the composite route which are not possible in a single material.
Inevitably, the demand for highly specialized combinations of properties in materials becomes more acute as the requirements imposed by increased sophistication in industry become more severe. For example, the electronics industry has been subject to increased demands for materials having specialized combinations of properties including expansivity, conductivity, density, strength and modulus as miniaturization of equipment coupled with higher power and the generation of more heat in smaller packages has proceeded.
It is recognized that copper is a metal characterized by high conductivity for heat and electricity which suffers drawbacks in terms of desired strength for many applications. Copper is relatively soft and malleable and is readily workable. Molybdenum is a metal characterized by high melting point, 2610.degree. C., whereas copper melts at 1083.degree. C. Molybdenum is strong and retains its strength at elevated temperatures, e.g., 1000.degree. C. Copper and molybdenum are essentially immiscible at any temperature. Forging temperatures for molybdenum are said to be generally between 1180.degree. and 1290.degree. C., a temperature range exceeding the melting point of copper. The expansivity of copper is much higher than that of molybdenum.
A review of the differences in properties and characteristics between molybdenum and copper would lead one skilled in the art to believe that molybdenum and copper could not be combined in a composite structure due to the great disparity in properties between the two metals. Such a composite has heretofore been unknown.
The present invention is based on the discovery of a method for producing composite sheet of copper and molybdenum. The invention also contemplates using dispersion strengthened copper as the copper layer to obtain further improvements in properties of the composite.